1. Field of the Invention
This invention relates to digital tape recorders/players such as, for example, digital video tape recorders.
2. Description of the Prior Art
Helical scanning magnetic tape recording is used in applications which require a high density of information to be recorded on a magnetic tape. When a tape is being recorded or replayed, it is wrapped in a gentle helical path around the surface of a rotary head drum which carries one or more magnetic heads. The tape advances around the drum by only a short distance during one revolution of the drum so that the heads record to, or replay from, a succession of parallel oblique tracks extending along and across the tape.
This recording technique is particularly suited for use in digital video recording. Here, a number of record/replay heads are mounted on the head drum to provide access to a group of adjacent recording tracks on the tape, with the video data being distributed between the different heads.
In digital video tape recorders it is necessary to take precautions against the loss of video data through tape damage, head malfunction, temporary "drop outs" and the like. These error handling techniques are particularly important in digital video tape recorders in which the video data is compressed before being recorded, for example using the so-called "MPEG" (Motion Pictures Experts Group) compression technique. This is because in techniques of this type, the reconstruction of a particular pixel may rely on a number of bytes of the compressed data. Similarly, the reconstruction of a frame of the output video signal often relies on data from temporally preceding or following frames, so the loss of a particular portion of the video data may not only affect that frame but also other frames which also depend on that data.
Error handling precautions often include providing error detecting and/or correcting codes in the recorded data, and error concealment systems in the replay processing. However, a particularly serious data loss (for example, the complete malfunction of one of the heads or a portion of serious tape damage extending across a large region of the tape) may defeat these error handling techniques.
It has been proposed to allow for the complete loss of selective groups of record/replay heads in a multi-head digital video tape recorder by spatially de-interleaving the image data into effectively "odd" and "even" samples along each line. The "odd" and "even" data streams are then separately compressed and are recorded on respective different groups of record heads. The technique facilitates error concealment when the image data is replayed, because even if an entire group of heads is lost, the data recorded by the other group will provide alternate samples of a reconstructed picture, from which the missing samples can be interpolated.
However, this technique is not ideal, because the de-interleaving process introduces distortions of its own, so that, for example, a 4:1 compression process performed after de-interleaving produces image quality corresponding to about a 5.4:1 compression performed directly on the source data.